Austenitic steel and articles made therefrom



United States Patent 3,017,266 AUSTENITIC STEEL AND ARTICLES MADE THEREFROM Joseph D. Murray, Shelfield, England, assignor to The United Steel Companies Limited No Drawing. Filed June 2, 1960, Ser. No. 33,374 9 Claims. (Cl. 75-128) Stress, tons/sq. in. Rupture life, Elongation,

hrs. percent My objectis to provide steels having substantially improved stress-to-rupture properties, as well as easy fabrication and weldability.

The base composition of steels according to this invention is as follows: chromium 12 to 18%, nickel 8 to 20%, manganese 3 to 10% and carbon 0.05 to 0.15%. The steels according to the invention also contain at least 0.5% molybdenum. They may contain tungsten as well, but no advantage is gained. The total content of Mo+2W is not more than 4%.

An important feature is that the steels contain from 0.005 to 0.075%, and preferably from 0.005 to 0.007%, boron.

In addition the steels contain at least 0.5% columbium with or without vanadium. Tantalum is the equivalent of columbium and may be used in its place. In making the steel I prefer to use commercial columbium, which is rarely free from tantalum. The columbium-l-tantalum content is preferably at least 10 times the carbon content. The total amount of columbium+tantalum and vanadium should not exceed 3%.

Titanium is not the equivalent of vanadium or columbium, but may be tolerated in minor amounts.

The steel may also contain any of the other incidental elements and impurities commonly present in chromiumnickel steel, and as used herein the statement that the balance of the steel is iron is not intended to exclude these incidental elements and impurities.

The reasons for these various ranges of alloy elements will now be considered. First, it must be understood that before the tubes or other articles made from the steels are put into service the steels are always heattreated for the purposes of homogenisation, stress-relief after fabrication and putting the steel into the most effective condition to resist creep. The heat treatment comprises heating at a high temperature, usually from 1920 to 2100 F., and is most conveniently a normalising treatment. It must also be understood that the steel should be fully austenitic.

The chromium content is determined by the need to impart adequate oxidation resistance on the one hand and to ensure that the steel is fully austenitic on the other hand.

There must be at least 8% nickel to ensure that the steel is fully austenitic and has edequate strength at high temperatures. Nickel is expensive,- and too much nickel reduces the ductility at high temperatures; there may be 7 3,017,266 Patented Jan. 16, 1962 up to 20% nickel, but there is no advantage in making the nickel content more than 14%.

The manganese imparts ductility, and for this purpose at least 3% and preferably 5% is required. Too much manganese lowers the melting point to such an extent as to increase the fabrication difiiculties, and therefore not more than 10% should be present.

The molybdenum is found to improve the life to rupture under stress. Tungsten has the same effect and if it is present the temperature of the heat treatment should be higher than if it is absent. Both molybdenum and tungsten enter into and form part of the base composition of the alloy.

Vanadium and columbium enter into solution at high temperatures and precipitate at lower temperatures. In choosing the most suitable composition regard must be had to the temperature of heat treatment, since at 1920 F. neither vanadium nor columbium is fully in solution; the part these elements play is then important in determining the life to rupture.

Boron is particularly advantageous in increasing the stress-to-rupture properties. As the boron content increases ditiiculty in hot-working and welding the steel also increases, so the boron content is preferably low. Reduction in the rupture strength brought about by the use of only a low boron content can be offset by the use of a relatively high columbium-l-tantalum content and the simultaneous use of a low carbon content.

The preferred steels according to the invention have the following composition:

The effect of the various elements is shown by the results of stress-rupture tests under creep conditions. Table 1 below refers to steels which were heat-treated at 2100 F. for /2 hour and air-cooled, and specimens of which were tested at 1290 F. under a stress of 15 tons per square inch. All the steels of nominal composition: 6% manganese, 16% chromium and 0.5% silicon. Steels l to 4 contained 8% nickel and steel 5 contained 10% nickel. The analyses of the other elements, and some of the stress-to-rupture properties of the steels, were as follows:

Life to Elonga- Steel No. G M0 V Cb B rupture tion perin hours cent It will be seen that steels l to 3 are not in accordance with the invention. In steels 1 and 2 both the columbium and the boron contents are too low, and both the life to rupture and the elongation are reduced. Steel 3 is boronfree, and although the high columbium content gives it a good life to rupture the absence of boron leads to much reduced elongation. Steel 4 shows the effect of including a small amount of boron in a steel otherwise substantially the same as steel 3.

The steels according to the invention may be fabricated and welded easily; possess good rupture properties; and can be heat-treated at temperatures within the range of 1920 to 2100 F., whereas existing steels of comparable properties must be heat-treated at distinctly higher temperaturcs and are of more expensive composition.

I claim:

1. Austenitic steel essentially consisting of chromium from 12 to 18%, nickel from 8 to 20%, manganese from 3 to 10%, carbon from 0.05 to 0.15%, boron ttrom 0.005 to 0.075%, columbium+tantalum at least 0.5%, vanadium up to 2.5%, the total of columbium-l-tantailum and vanadium not exceeding 3%, molybdenum from 0.5 to 4% and tungsten up to 3.5% the total of molybdenum and tungsten not exceeding 4%, and the balance iron.

2. Steel according to claim 1 in which the boron content does not exceed 0.007%.

3. Steel according to claim 1 in which the nickel content does not exceed 14%.

' 4. Steel according to claim 1 in which the columbium-l-tantalum content is at least 10 times the carbon content.

I 5. Austenitic steel essentially consisting of 0.08 to 0.12% of carbon, 6.0 to 6.5% of manganese, 0.4 to 0.6% of silicon, 9 to 11% of nickel, 14 to 16% of chromium, 0.9 to 1.1% of molybdenum, 0.2 to 0.6% of vanadium, 0.8 to 1.2% of at least one metal selected from the group consisting of columbium and tantalum, 0.005 to 0.007% of boron and the balance iron.

6. Superheating tubing made of an austenitic steel essentially consisting of 0.08 to 0.12% of carbon, 6.0 to 6.5 of manganese, 0.4 to 0.6% of silicon, 9 to 11% of nickel, 14 to 16% of chromium, 0.9 to 1.1% of 4 molybdenum, 0.2 to 0.6% of vanadium, 0.8 to 1.2% of at least one metal selected from the group consisting of columbium and tantalum, 0.005 to 0.007% of boron and the balance iron,

7. Steam pipes made of an austenitic steel essentially consisting of 0.08 to 0.12% of carbon, 6.0 to 6.5% of manganese, 0.4 to 0.6% of silicon, 9 to 11% of nickel, 14 to 16% of chromium, 0.9 to 1.1% of molybdenum, 0.2 to 0.6% of vanadium, 0.8 to 1.2% of at least one metal selected from the group consisting of columbium and tantalum, 0.005 to 0.007% of boron and the balance Iron.

8. Steam heaters made of an austenitic steel essentially consisting of 0.08 to 0.12% of carbon, 6.0 to 6.5% of manganese, 0.4 to 0.6% of silicon, 9 to 11% of nickel, 14 to 16% of chromium, 0.9 to 1.1% of molybdenum, 0.2 to 0.6% of vanadium, 0.8 to 1.2% of at least one metal selected from the group consisting of columbium and tantalum, 0.005 to 0.007% of boron and the balance iron.

9. Tailpipes of gas turbine engines for aircraft made of an austenitic steel essentially consisting of 0.08 to 0.12% of carbon, 6.0 to 6.5% of manganese, 0.4 to 0.6% of silicon, 9 to 11% of nickel, 14 to 16% of chromium, 0.9 to 1.1% of molybdenum, 0.2 to 0.6% of vanadium, 0.8 to 1.2% of at least one metal selected from the group consisting of columbium and tantalum, 0.005 to 0.007% of boron and the balance iron,

References Cited in the file of this patent UNITED STATES PATENTS 2,750,283 Loveless June 12, 1956 2,772,155 Eisermann et a1 Nov. 27, 1956 

1. AUSTENITIC STEEL ESSENTIALLY CONSISTING OF CHROMIUM FROM 12 TO 18%, NICKEL FROIM 8 TO 2/%, MANGANESE FROM 3 TO 10%, CARBON FROM 0.05 TO 0.15%, BORON FROM 0.005 TO 0.07K%, COLUMBIUM + TANTALUM AT LEAST 0.5% VANADIUM NOT EXCEEDING 3%, THE TOTAL OF COLUMBIUM + TANTALUM AND VANADIUM NOT EXCEEDING 3%, MOLYBEDNUM FROM 0.5 TO 4%, AND TUNGSTEN UP TO 3.5%, THE TOTAL OF MOLYBDENUM AND TUNGSTEN NOT EXCEEDING 4%, AND THE BALANCE IRON. 